Experimental Study And Numerical Simulation Of Liquid Film Flow Characteristics For Dip Coating Process

Dip coating is a process that a clean substrate is dipped into the fluid had been prepared,and then lifted out at a constant speed, thus the effects of viscous force, gravity and surface tension keep the fluid deposited on the substrate surface, and finally forms a uniform coating film after drying and curing. The coating film has been used widely in industry, medicine,electronic and digital fields. With the development of advanced film materials, the coating film is also gradually developed from traditional protective and decorative film to functional film materials. In this paper, the dip coating process was regarded as research background, and liquid film flow problem was studied by experiment and numerical simulation for silicone oil fluids and PAA gels had different properties, which had important theoretical and practical significance for optimization of the parameterized control and improvement of coating film quality in dip coating.The rheological properties and surface tension of test fluids were measured with rotational rheometer and surface tension meter respectively, and the fitted parameters were obtained by choosing appropriate models to fit rheological curves in order to lay a foundation for later experiment and numerical simulation. The rheological curves of fluids showed that silicone oil A was Newtonian fluid, the rheological behavior of silicone oil B, C and D could be fitted with Carreau-Yasuda(CY) model, while PAA gels with different mass fraction were yield stress fluids,whose rheological curves could be fitted with Herschel-Bulkley(HB) model. The analysis of surface tension illustrated that the surface tension of silicone oil fluids were smaller than PAA gels, which was beneficial to wetting the substrate and forming liquid film.During dip coating process, experimental study was carried out with dip coater under different process conditions, the liquid film thickness deposited on the plate substrate was measured using capacitance method, and the evolution of liquid film flow and free surface morphology were recorded with photographs for silicone oil fluids and PAA gels. The purpose was to investigate the effects of plate substrate roughness, withdrawal speed, rest time and fluidviscosity on the liquid film thickness and free surface morphology in detail. The experimental results showed that the influence of rest time on liquid film thickness could be neglected when it was greater than 3 min, the free surface in the meniscus region exhibited a depression due to the fluid entrained along the substrate could not be compensated timely for silicone oil fluid, and a bump was presented on the free surface for PAA gel under given conditions, moreover the liquid film thickening, the depression gradually deepened and the bump curvature increased with the substrate roughness, withdrawal speed and fluid viscosity increasing.First the physical model concerning to the drag-out of the fluid by plate substrate was established, and structured meshes were generated using pre-processing software Gambit, then the appropriate physical parameters, boundary conditions and calculation methods were set up to simulate accurately the free surface and flow field at different coating parameters using finite element analysis software Polyflow for the two kinds of fluids. Finally the numerical results of liquid film thickness, free surface and flow field were displayed by post-processing software Fieldview. In the paper, the numerical results were compared well with experimental results within a certain error range, which demonstrated that the numerical methods could be used to optimize design research. The numerical results about free surface and velocity flow field showed that during the substrate withdrawn from the bank, a part of fluid adjacent to the substrate was entrained along the plate substrate and formed liquid film layer, the other part fluid returned to the bank and simultaneously a stagnation point was formed in the dynamic region of free surface and a recirculation was generated in the flow field.Surface tension, gravity and viscous force played an important role in dip coating process,the rotational rheometer was modified optimally to measure the force of the substrate withdrawn from the fluid for investigating the liquid film flow characteristics in detail. The force analysis was carried out to explain the effect of the force on film flow characteristics and liquid film thickness. Fluid surface tension values were obtained simultaneously, and were compared with that measured by Wilhelmy method, and a maximum of 7% difference was generated, which llustrated that using the dip coating method of modified rotational rheometer was feasible to measure surface tension.